The disproportionation of toluene involves a well known transalkylation reaction in which toluene is converted to benzene and xylene in accordance with the following reaction: ##STR1## Reaction (1) is mildly exothermic.
Mordenite is one of a number of catalysts commonly employed in the transalkylation of alkylaromatic compounds. Mordenite is a crystalline aluminosilicate zeolite having a network of silicon and aluminum atoms interlinked in its crystalline structure through oxygen atoms. For a general description of mordenite catalysts, reference is made to Kirk-Othmer Encyclopedia of Chemical Technology, 3rd Edition, 1981, under the heading "Molecular Sieves", Vol. 15, pages 638-643. Mordenite, as found in nature or as synthesized, typically has a relatively low silica to alumina mole ratio of about 10 or less. Such conventionally structured mordenite catalysts are commonly employed in the disproportionation of toluene. However, mordenite catalysts having substantially lower alumina content are also employed in the disproportionation of toluene.
The aluminum deficient mordenite catalysts have a silica/alumina ratio greater than 10 and may sometimes range up to about 100. Such low alumina mordenites may be prepared by direct synthesis as disclosed, for example, in U.S. Pat. No. 3,436,174 to Sand or by acid extraction of a more conventionally prepared mordenite as disclosed in U.S. Pat. No. 3,480,539 to Voorhies et al.
U.S. Pat. No. 3,780,122 to Pollitzer discloses the transalkylation of toluene using a mordenite zeolite having a silica/alumina ratio greater than 10 which is obtained by acid extraction of a mordenite zeolite having a silica/alumina ratio of less than 10. The silica/alumina ratio may range up to about 100 and preferably is at least about 15. The yield in the Pollitzer process is severely affected by water in the toluene feedstock. As shown in Table II, even a very small amount of water (15 ppm) reduces toluene conversion substantially and the patent designates an upper limit of 25 ppm water in the feedstock.
The disproportionation of toluene feedstocks may be carried out at temperatures ranging from about 200.degree. C. to about 600.degree. C. or above and at pressures ranging from atmospheric to perhaps 100 atmospheres or above. However, the catalyst itself may impose constraints on the reaction temperatures in terms of catalyst activity and aging characteristics. In general, the prior art indicates that while relatively high temperatures can be employed for the high aluminum mordenites (low silica to alumina ratios) somewhat lower temperatures should be employed for the low alumina mordenites. Thus, where mordenite catalysts having high silica/alumina ratios have been employed in the transalkylation of alkylaromatics, it has been the practice to operate toward the lower end of the temperature range. It is also a common practice in this case to promote the catalyst with a catalytically active metallic content. For example, U.S. Pat. No. 3,476,821 to Brandenburg et al discloses disproportionation reactions employing mordenite catalysts having a silica/alumina ratio within the range of 10-100 and preferably within the range of about 20-60. Here desired temperature ranges are said to be from about 400.degree.-750.degree. F. and preferably 450.degree.-640.degree. F. Metal promoters are said to substantially increase activity and catalyt life.
It is conventional practice to supply hydrogen along with toluene to the reaction zone. While the disproportionation reaction (1) is net of hydrogen, the use of a hydrogen co-feed is generally considered to prolong the useful life of the catalyst, as disclosed, for example, in the above patent to Brandenburg. The amount of hydrogen supplied, which normally is measured in terms of the hydrogen/toluene mole ratio, is generally shown in the prior art to increase as temperature increases. For example, while the patent to Pollitzer discloses a range for the hydrogen/toluene mole ratio of 2-20 corresponding to the broad temperature range of 200.degree.-480.degree. C., the specific examples in Pollitzer of operating at temperatures ranging from 300.degree.-400.degree. C. employ a hydrogen/toluene mole ratio of 10.
The use of mordenite catalysts of high silica/alumina ratio in toluene disproportionation is also disclosed in U.S. Pat. No. 3,915,895 to Suggitt et al. The silica/alumina mole ratios proposed in Suggitt range from 10 to about 100 (preferably 12-80 and more preferably about 25 to 50). The catalysts for which experimental information is given in Suggitt had silica/alumina ratios of 18 and 39. At the disproportionation conditions employed (550.degree. F. and 200 or 800 psig.), neither catalyst showed particularly good activity although the higher alumina catalyst promoted with silver was better than the unpromoted catalyst.
Bhavikatti et al, "Toluene Disproportionation over Aluminum-Deficient and Metal-Loaded Mordenites. 1. Catalytic Activity and Aging", Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 102-105, discloses toluene disproportionation at 400.degree. C. over mordenite catalysts of silica/alumina mole ratios of 12, 16, 23, 32, and 61. The tests reported in Bhavikatti were carried out at atmospheric pressure and with a WHSV of 1. As the silica/alumina mole ratio is increased, catalyst activity substantially decreased while aging quality increased. That is, the aging rates were lower. Based upon short term aging studies, the best silica/alumina mole ratio appeared to be 23. Catalyst decay was also supressed by loading the mordenite with nickel.
U.S. patent application Ser. No. 826,848 filed Feb. 6, 1986, by James R. Butler and Kevin P. Menard discloses disproportionation of a toluene containing feedstock employing an aluminum deficient mordenite catalyst under relatively severe disproportionation conditions. The mordenite catalyst has a silica/alumina mole ratio of at least 30 and preferably a silica/alumina ratio within the range of 40-60. The feedstock may be supplied to a reaction zone containing the mordenite catalyst at rates providing relatively high space velocities. The toluene weight hourly space velocity (WHSV) may be greater than 1. Hydrogen is also supplied to the reaction zone at a rate to provide a hydrogen/toluene mole ratio within the range of 3-6. The reaction zone is operated at a temperature in the range of 370.degree.-500.degree. C. and a hydrogen pressure of at least 500 psig to effect disproportionation of the toluene to benzene and xylenes. More specific reaction conditions include a temperature within the range of 400.degree.-480.degree. C., a hydrogen pressure of about 600-800 psig, and a mole ratio of hydrogen to toluene of about 4. The preferred catalyst is hydrogen mordenite having a silica/alumina ratio of about 48.
The toluene feedstock used in the process of application Ser. No. 826,848 need not be dried before supplying it to the reaction zone. Thus, toluene feedstock having a water content in excess of 25 ppm can be applied directly into the reaction zone. The Butler et al application further discloses passing a preflush gas to the reaction zone prior to initiating the disproportionation reaction. The preflush gas is heated to a temperature sufficient to strip water from the catalyst so that it is substantially dehydrated by the time the toluene feed is started. This enables the disproportionation process to be carried out initially at a somewhat lower temperature without a reduced toluene conversion than would otherwise be the case. As the disproportionation process continues, the temperatures progressively increase to maintain the toluene conversion at the desired level, typically about 80% of theoretical.